A Logitech or Microsoft webcam can fit the bill.
Logi's solitary Mac webcam
is optimized for the Mac: its autofocus and auto-exposure is implemented in hardware, unlike the PC version. The flip side is this: despite being an "HD" camera, no meaningful Mac programs can use HD. They're limited to the VGA resolution of Mac OS X's webcam support. Also, this camera is strictly standards-based (UVC), so it will work without drivers in Windows 7 and Vista, but not in XP without drivers. If you need to know, I can test this, since I have this camera and various Windows VMs.
Alternatively, any camera that advertises as being compatible with Windows 7 or Vista is compatible with Mac OS X, without any drivers. The downside is that since most cameras implement exposure and focus in the Windows drivers, so you may not get the best images.
All webcams work just as microphones. I do this all the time. (Technically, a modern webcam is two devices in one housing: a USB camera and a USB microphone. They're supported in software totally independently of each other.)